The present invention relates to an implantable electrical device, e.g., an implantable medical device such as an implantable cochlear stimulation system, which receives its operating power and/or which receives recharging power from an external (non-implanted) power source.
Implantable electrical devices are used for many purposes. A common type of implantable device is a tissue stimulator. A tissue stimulator includes one or more electrodes in contact with desired tissue. An electrical stimulation current is generated by the stimulator and applied to the tissue through the electrode(s).
In order for an implanted device to perform its intended function, e.g., to generate an electrical stimulation current, it needs a power source. Some implanted devices, e.g., cardiac pacemakers, employ a high capacity battery that has sufficient power stored therein to provide operating power for the device for several years. Other implanted devices, e.g., a cochlear stimulation system, do not use an implanted power source but rather receive a continuous stream of power from an external source through an rf (radio frequency) or inductive link. Yet other implanted devices include a rechargeable power source, e.g., a rechargeable battery, that must be regularly recharged, e.g., once a day, or 2-3 times per week, from an external source in order for the implanted device to operate. The present invention is intended for use with the latter two types of implanted devices, e.g., those that receive a continuous stream of operating power from an external source, and/or those that must receive power at regular intervals in order to recharge an implantable power source.
Power is typically coupled to an implanted device through inductive coupling. Inductive coupling advantageously avoids the use of wires that must pass through or penetrate the skin. With inductive coupling, an external coil receives an ac power signal. An implanted coil connected to, or forming part of, the implantable device, is placed in close proximity to the external coil so that magnetic flux generated by the ac power signal in the external coil induces an ac power signal in the second coil, much like the primary winding of a transformer couples energy to a secondary winding of the transformer, even though the two windings are not directly connected to each other. When inductively coupling power to an implanted device in this manner, an optimum power transfer condition exists only when there is a good impedance match between the implant device and the external device. While impedance matching schemes can and have been used in the external device, such matching schemes are only effective for a given distance between the external coil and the implant coil, and for a given load attached to the implant device.
Unfortunately, neither the load associated with the implant device nor the separation distance between the external coil and the implant coil are constants. Each of these parameters are, in practice, variables, that may vary, e.g., from 3-to-15 mm for the separation distance, and 20 to 300 ohms for the load. As a result, optimum power transfer between the external device and implant device is rarely achieved. Thus, a less than optimum power transfer condition exists and much of the energy sent to the external coil is lost. What is needed, therefore, is a way to assure that optimum power transfer conditions exist between the external coil and implant device at the time a power transfer is made.
For many implant devices, optimum power transfer has heretofore generally not been a serious concern inasmuch as the external device (which has generally comprised a relatively large device that is worn or carried by the patient) has been viewed as having a potentially infinite power source (through recharging and/or replacing its battery). Unfortunately, however, transferring large amounts of power without concern for how much power is lost is not only inefficient, but may create regulatory problems. That is, most regulatory agencies stipulate the power levels that may be used with an implant device.
Further, new generation external devices are being made smaller and smaller to accommodate the needs and desires of the user. For example, a behind-the-ear (BTE) external device may be used with an implantable cochlear stimulator (ICS). Such a BTE external device is about the same size as a conventional behind-the-ear hearing aid. Such smaller devices, as a practical manner, do not have a potentially infinite power source, but must be powered using a small button battery, or equivalent. Such a small battery must provide power for both the external unit and the implant unit, and achieving an efficient power transfer is a key element in assuring a long battery life.
It is known in the art, see, e.g., U.S. Pat. No. 4,654,880, to include the external coil and implant coil (as coupled to each other based on a given separation distance and load) in the oscillator circuit that sets the frequency of the signal that is coupled between the external coil and implant coil. Such circuit is somewhat self-compensating because as the transfer efficiency starts to go down (e.g., because the separation distance changes, or because the load changes) the frequency of the signal used to couple energy into the implant coil automatically changes in a direction that tends to retune the coupled coils so that the energy transfer becomes more efficient.
It is also known in the art, see, e.g., U.S. Pat. No. 5,179,511, to use a self-regulating Class E amplifier, combined with current feedback, to better control the frequency of the coupling signal so as to achieve a more optimum energy transfer.
Disadvantageously, changing the frequency of the signal coupled into the implant circuit may also create regulatory problems. That is, regulatory agencies are typically very strict about the frequencies of signals that are allowed to be transmitted, even if only transmitted over short distances.
One technique known in the art for optimally transferring power is through the use of a DC-to-DC converter. Disadvantageously, stability problems may arise when using a DC-to-DC converter. More particularly, switching regulators, a common form of DC-to-DC converters, are prone to xe2x80x9cbistabilityxe2x80x9d, as discussed in the article: xe2x80x9cSource resistance: the efficiency killer in DC-DC converter circuitsxe2x80x9d, which article is attached hereto as Appendix A and is incorporated herein by reference.
In view of the above, it is evident that what is needed is a transmission scheme for use with a medical implant device that optimally transfers power to the implant device from an external device at a fixed frequency, i.e., that transfers power into the implant device from the external device with minimum power loss.
The present invention addresses the above and other needs by providing a fixed frequency external power source that is inductively coupled with an implanted device. Unlike prior art implanted devices, however, the implant device of the present invention utilizes an electronic impedance transformer as part of the load circuit in the implant device. Such electronic impedance transformer stabilizes, or makes constant, the load resistance. While the impedance seen looking into the external coil is still very much a function of the coil separation, and hence may not be optimal (this impedance follows a parabolic shaped loss curve, well known in the art, as a function of coil separation distance), it is now possible, with an adjustable stabilized load resistance (made possible by the impedance transformer of the present invention) for a smart external device to measure the impedance seen looking into the external coil (which impedance includes both the coil separation loss and the stabilized load resistance made possible by the invention) and vary the internal impedance transformer to achieve an overall better power transfer. Hence, the invention makes possible the proper voltage and current ratio (resistance) to exist, so that the coil set, i.e., the external coil and the implanted coil, are loaded with the xe2x80x9cbest availablexe2x80x9d load under the circumstances. Such best possible load, in turn, minimizes mismatch losses from the inductive link between the external coil and the implant coil, and allows wide ranges in the voltage and load resistance and coil separation, while at the same time maintains a best possible load condition.
The present invention is especially applicable to fully implantable cochlear stimulation systems. A representative fully implantable cochlear stimulation system is disclosed, e.g., in U.S. Pat. No. 6,067,474 and/or in U.S. patent application Ser. No. 09/404,966, filed Sep. 24, 1999, which patent and patent application are incorporated herein by reference. In a fully implantable system (FIS), the FIS preferably operates using power from an implanted power source, such as a rechargeable battery, which power source must be periodically recharged by transferring large amounts of power to the implant device. However, the FIS must also be able to operate, from time to time or in the event of a battery or other failure, using an external behind-the-ear (BTE) unit, or other external unit, which requires a power transfer at much lower power levels than are needed for recharging. That is, in the FIS, during one mode of operation, a relatively large power level must be transferred for charging the implanted power storage element, e.g., a rechargeable battery. However, in another mode of operation, the implant is operated and powered from a BTE unit, or other external unit, during which mode a relatively small power level is transferred to the implant device. The ratio of these power levels may be, e.g., about 30 to 1. Unless the coil set, i.e., the external coil and implanted coil, are altered between these different load conditions, a mismatch loss on the order of 14dB may occur, which mismatch may reduce the transfer efficiency from about 70% to about 3%! The present invention advantageously eliminates such a mismatch loss.
In accordance with one aspect of the invention, a time-varying impedance transformer is utilized to make the mismatch loss constant. The control of the mismatch is determined by the load impedance, once all other components are fixed. However, because an implant device of the type with which the present invention is used may require a range of output voltages, and output currents, the effective load resistance is not equivalent to a single load resistance, but rather varies as a function of time dependent upon the required circuit operation. The time-varying impedance transformer provided in the implant device by the invention thus operates to stabilize (make constant insofar as possible) the ratio of output voltage and output current as seen by the coil set, thereby rendering the mismatch loss constant, even though the individual output voltages and output currents do vary.
In accordance with another aspect of the invention, a switching regulator circuit is employed as the time varying impedance transformer. Advantageously, a switching regulator circuit provides for the efficient transfer of electrical power from one voltage level to another. A switching regulator operates as a DC-to-DC impedance transformer. That is, at its input, the switching regulator consumes the required current at the source voltage level, and transforms the current to a new level at a different output voltage. Since energy is neither created nor destroyed, the switching regulator functions as a power transformer, with some loss occurring (as determined by the converter efficiency). Hence, in accordance with the present invention, a switching regulator included as part of the implant circuitry is controlled in an appropriate manner so that the resulting impedance transforming property of the switching regulator reduces mismatch loss variations.
It is thus a feature of the present invention to provide an implantable medical device, e.g., an implantable cochlear stimulator or other implantable neural stimulator, that employs a switching regulator circuit as part of the implanted circuitry. The switching regulator is controlled, as energy is inductively coupled into the implant circuitry through a coil set that includes an external coil and an implanted coil, to operate as a varying impedance transformer. More particularly, the impedance is varied so as to minimize mismatch losses as seen at the power source, thereby improving the power transfer efficiency into the implant device.
It is a further feature of the invention to provide an implantable time-varying impedance transformer wherein there are no circuit value or wiring changes needed to handle varying output load impedance. Rather, all control is entirely electronic.
One advantage of the invention is that the frequency of the carrier signal (the signal applied to the external coil) is fixed, thereby avoiding regulatory or other problems incident to using variable frequency carrier signals.
An additional advantage of the invention is that the effective DC load resistance of the implanted circuitry (output voltage divided by output current) is transformed to effect an AC circuit mismatch loss, so that the lowest insertion loss of the coil set (i.e., the highest power transfer efficiency between the external coil and implanted coil) may be utilized.
Still another advantage of the invention is that the voltage transform ratio of the coil set is, within certain practical constraints, relatively independent of the voltage at the output load.
Another advantage of the invention, when used in combination with a smart external power source that can regularly measure the impedance as seen looking into the external coil and communicate this measured impedance to the implant device, is that the impedance transforming process that occurs in the implant device, acting upon the measured impedance information obtained from the smart external device, may also be used to compensate for variations in transfer efficiency that occur due to coil separation.